1. Field of the Invention
The present invention relates to a process for converting silicon metal and methanol to trimethoxysilane and in particular to a process in which a mixture containing trimethoxysilane and methanol (in the azeotropic ratio of 55% trimethoxysilane to 45% methanol by weight) is recycled to a reactor without employing a separate azeotropic or extractive distillation step.
2. Prior Art
The reaction between silicon metal and alcohol to produce alkoxysilanes and silicates is well established. As early as 1949, U.S. Pat. No. 2,473,260 described a process for the preparation of methyl silicates from methanol and silicon-copper masses. Subsequently, U.S. Pat. No. 3,072,700 taught the preparation of alkoxysilanes from silicon metal and alcohol in a fluidized bed reactor.
Patents on the production of tetraalkylorthosilicates include U.S. Pat. No. 4,288,604 and Japanese Pat. No. 1979-163529. A patent covering the production of trialkoxysilanes is U.S. Pat. No. 3,775,457. One of the problems associated with such processes is the difficulty of removing the unreacted alcohol from the desired silane.
In Japanese Laid Open Application No. 1980-11538 is described a process to produce trimethoxysilane wherein the unreacted methanol in the product is removed by breaking the trimethoxysilane-methanol azeotrope by adding a third component, e.g., hexane, in an amount proportional to the amount of methanol present and then distilling to remove the methanol as the hexane-methanol azeotrope.
U.S Pat. No. 4,761,492, discloses a process in which methanol is separated from crude product containing trimethoxysilane using extractive distillation with tetramethoxysilane as the solvent. This process requires a relatively large quantity of tetramethoxysilane for extractive distillation. That is, the ratio of tetramethoxysilane to the crude product must be equal to or greater than 2:1. This requirement means that the extraction column must handle a flowrate of at least three times the desired production rate of the crude product. Therefore, the column diameter and number of trays (approximately 50 to 60) are in excess of what would otherwise be needed for the extraction column. Another problem with the process is the difficulty of obtaining high purity trimethoxysilane (i.e., greater than 99%) given the large ratio of tetramethoxysilane. Because of this ratio, separation of high purity trimethoxysilane is difficult to achieve even in a second distillation column.
It is known in the art that when methanol and silicon metal are reacted to make trimethoxysilane, large quantities of unreacted methanol are generally present in the reactor product. While the industry has long desired and felt that there was a need to recycle unreacted methanol to the reactor for economic reasons, it was generally believed not to be possible or feasible without separation of the methanol from a crude product. It has been generally taught in the art that removal of this unreacted methanol is required prior to utilizing the trimethoxysilane in order to avoid the reaction of methanol with trimethoxysilane to form hydrogen and tetramethoxysilane. For example, in U.S. Pat. No. 4,727,173 (Column 3, lines 48-51), it is stated, "If the crude product is recycled to the reactor with the trimethoxysilane unremoved, the trimethoxysilane will likely react further with the methanol to produce tetramethoxysilane." And further, Japanese Laid Open Application No. 1980-11538 (page 5, lines 12) states "it is necessary to separate and remove unreacted methanol rapidly from the reaction mixture in order to obtain methoxysilane, particularly trimethoxysilane, at a high yield." Therefore, the manufacture of trimethoxysilane from silicon metal and methanol required extractive distillation; or, alternatively, subsequent, separate azeotropic distillation using a third component, such as hexane, to break the azeotrope.
It is well known in the art that methanol and trimethoxysilane form a relatively low boiling (i.e., a normal boiling point of about 62.5.degree. C.) azeotrope which is about 55 wt % trimethoxysilane and 45 wt % methanol. Similarly, methanol has a normal boiling point of about 64.5.degree. C., while trimethoxysilane has a normal boiling point of about 84.degree. C. Since the azeotrope has the lowest boiling point, it is not possible to separate a stream containing methanol and trimethoxysilane into a stream of pure methanol and a stream of a pure trimethoxysilane by simple distillation alone.
Because methanol and trimethoxysilane form a low boiling azeotrope which is about 55 wt % trimethoxysilane and 45 wt % methanol, and methanol and trimethoxysilane react at elevated temperatures, it was believed that "azeotrope recycle" would result in large trimethoxysilane losses and poor selectivity. Selectivity here refers to the amount of trimethoxysilane relative to the amount of tetramethoxysilane contained in the reactor product. Further, it was believed even if azeotrope recycle were to be attempted, the volume of the recycle feed would be so large that the equipment required for handling would be costly and unwieldy. Also, effectively handling such volume would raise concerns about safety and controlability of the process. Until the present invention, recycle has been impractical and unknown.
Accordingly, a need continues to exist for a commercially attractive process to recover trimethoxysilane relatively free from methanol and tetramethoxysilane.